Sign support structure



FIG. 1 is a perspective view of my design for sign support structure;

FIG. 2 is a front, elevational view thereof;

FIG. 3 is a rear, elevational view thereof;

FIG. 4 is a side elevational view thereof, with the other side being a mirror image;

FIG. 5 is a top, plan view thereof; and,

FIG. 6 is a bottom plan view. 

I claim the ornamental design for a sign support structure, as shown and described. 